算法思想：

找一个子串是否在另一个串中出现

#include<string>
string s1 = "abcdefghi";
string s2 = "def";
int BF()
{int i = 0;int j = 0;while (i < s1.size() && j < s2.size()){if (s1[i] == s2[j]){i++;j++;}else{i = i - j + 1;j = 0;}}if (j == s2.size()){return i - j;}else{return -1;}
}
int main()
{int pos = BF();cout << pos << endl;return 0;
}